The project is concerned with components of the prothrombin complex. Improved procedures for the separation of components are to be worked out with the main purpose being to increase yield, purity, and to simplify the work. Both human and bovine materials are to be used. The plan calls for a continuation of studies on purified bovine Factor IX. It is to be digested with thrombin so as to produce an inhibitor and a peptide(s). If the hypothetical peptide can be isolated it is to be characterized. Thrombin is to be produced with the purpose of studying crystals to determine the tertiary structure of thrombin. The purified thrombin converts Protein C to autoprothrombin II-A and further information on that process is to be obtained. The activation of autoprothrombin IIIm (Factor X Beta) is quite different from that of autoprothrombin III (Factor X), but more information on that point is needed, and especially with respect to the place of Factor VII. Autoprothrombin II-A, in purified form, inhibits Factor Xa. By using 25% sodium citrate solutions it should be possible to demonstrate and describe the nature of the inhibition. Under those conditions there will be no possibility to complicate the interpretation with questions about inhibition of Ac-globulin. Until now there have been only a few studies related to the generation of acarboxy prothrombin, and the project is to include the task of obtaining further information on the conversion of acarboxy prothrombin to thrombin with physiological procoagulants.